In general, a thermally sensitive recording medium which obtains a recorded image utilizing a color developing reaction by the heating of a colorless or pale colored basic leuco dye with a color developing agent has advantages that the coloring is very clear, noiseless during the recording process and the equipment is relatively cheap, compact and maintenance free, and is widely applied in the field of a facsimile, a computer field and a recorder for various measuring instruments. Further, currently, a use as an output medium for various printers or plotters such as a handy terminal for outdoor measurement or a delivery slip, besides a use for a label or a ticket, are rapidly expanding. These usages are characterized in being often used outdoors, therefore, qualities and property characteristics for being used under a more serious environment than conventional uses, for example, water and high humidity caused by rain, sunshine or high temperatures in a car during midsummer season. Furthermore, it is necessary to provide a good printing aptitude in these different uses, especially, when used as a delivery slip, a good seal-ability is an important quality.
Regarding an improvement in water-resistance, for example, in patent document 1, a method of adding an isocyanate compound to an adhesive such as polyvinylalcohol is disclosed. In the meanwhile, a method to improve the water-resistance by using a hydrophobic resin emulsion such as a vinyl acetate emulsion, acrylic emulsion or SBR latex as an adhesive for a thermally sensitive layer is known, however, since these methods have a problem of causing the accumulation of dregs on a head or sticking during the recording process, actual use has not been sufficiently accomplished yet. Further, a method of using a composite of colloidal silica and an acrylic polymer as an adhesive is proposed in patent document 2 and a method of using a self-crosslinkable acrylic emulsion and colloidal silica is proposed in patent document 3. However, these methods cannot achieve a water-resistance satisfying outdoor use and the problem of accumulation of dregs on a head is not suppressed sufficiently.                [Patent Document 1] JP-A-S55-159993 publication        [Patent Document 2] JP-A-H09-207435 publication        [Patent Document 3] JP-A-H09-266711 publication        
In patent document 4, a water-resistant coating composition prepared by blending colloidal silica with an aqueous emulsion containing a copolymer obtained by the emulsion-polymerization of a vinylsilane and an acrylic monomer is disclosed.                [Patent Document 4] JP-B-H03-47699 publication        